This invention relates to galvanic sensors, and more particularly to solid oxide electrolyte galvanic sensors for exhaust gases.
A solid electrolyte galvanic sensor can be used to measure the chemical content of combustion gases produced in an internal combustion engine. The sensor produces an output voltage that can be used as a direct measure of oxygen or unburned combustibles in the combustion gases. It can be used in monitoring and controlling the combustion process. U.S. Pat. Nos. 3,616,274, Eddy and 3,844,920, Burgett et al. disclose sensors of this type.
The sensor can be a tube of oxygen-ion-conductive ceramic, such as zirconia, having inner and outer electrodes. The inner electrode is exposed to a reference gas of known oxygen partial pressure, as for example air. The outer electrode is exposed to the combustion gases. If the combustion gases are from a fuel-lean air-fuel mixture, the sensor has a low output voltage. If the combustion gases are from a fuel-rich air-fuel mixture, the sensor has a high output voltage. The change from low to high output occurs within a narrow range of air-fuel mixtures that are substantially stoichiometric in composition. Sensor output voltage can thus be used to detect whether a lean, rich or stoichiometric air-fuel mixture was combusted.
Sensor output voltage also varies with temperature, particularly when analyzing exhaust gases from fuel-lean air-fuel mixtures. At a fixed exhaust gas composition and below about 800.degree. C., sensor output voltage generally decreases with increasing temperature. Above about 800.degree. C., output voltage no longer decreases. It becomes relatively constant and in fact increases slightly. However, for purposes of this invention, sensor output voltage is considered to be substantially independent of temperature above about 800.degree. C. The aforementioned U.S. Pat. No. 3,616,274 Eddy avoids temperature effects by maintaining the sensor at a constant operating temperature. I have recognized that a temperature dependent resistance, connected across the sensor output, can be used to compensate for temperature effects. Moreover, I have found how to provide such a resistance in a simple and effective manner. The solid electrolyte of my sensor is doped to have a predetermined thermally dependent electronic conductivity. The electronic conductivity provides an internal electronic resistance that is electrically in shunt across the sensor output. The electronic resistance decreases with increasing temperatures. Such doping can provide a sensor output voltage that is substantially independent of temperature above temperatures as low as about 450.degree. C.